Universality of free fall from the orbital motion of a pulsar in a stellar tripl.pdf表明爱因斯坦对引力的洞察仍然占据着主导地位，即使是在宇宙所能提供的最极端的情况下。除去所有的空气，一个锤子和一根羽毛将以同样的速度下降——这是16世纪晚期伽利略探索的一个概念，由阿波罗15号宇航员大卫斯科特( David Scott)在月球上做了一个著名的解释。尽管牛顿物理学的基石，爱因斯坦的引力理论却解释了这是如何以及为什么是这样的。到目前为止，爱因斯坦的方程式已经通过了所有的测试，从仔细的实验室研究到对太阳系中行星的观察。但是，爱因斯坦的广义相对论的替代品预言，具有极强引力的致密物体，如中子星，与质量较差的物体有一点不同。这些不同的理论预测，这种差异将是由于一个紧凑的物体所谓的引力结合能——引力能把它结合在一起。参与此项研究的科学家有来自荷兰阿姆斯特丹大学（University of Amsterdam）、荷兰无线电天文学研究所(Netherlands Institute for Radio Astronomy, ASTRON)、澳大利亚斯威本科技大学（Swinburne University of Technology）、澳大利亚研究委员会重力波发现卓越中心（The Australian Research Council Centre of Excellence for Gravitational Wave Discovery, OzGrav)、美国威斯康星大学密尔沃基分校（University of Wisconsin-Milwaukee）、美国西弗吉尼亚大学（West Virginia University）、美国重力波和宇宙学中心（Center for Gravitational Waves and Cosmology, Morgantown, WV, USA）、格林·班斯天文台（Green Bank Observatory）、美国国家射电天文台（National Radio Astronomy Observatory）、加拿大 不列颠哥伦比亚大学（University of British Columbia, Vancouver, British Columbia, Canada）更多信息请注意浏览原文或者相关报道。

July 4, 2018, Green Bank Observatory

Artist impression of the triple star system PSR J0337+1715, which is located about 4,200 light-years from Earth. This system provides a natural laboratory to test fundamental theories of gravity. Credit: NRAO/AUI/NSF; S. Dagnello

Einstein's understanding of gravity, as outlined in his general theory of relativity, predicts that all objects fall at the same rate, regardless of their mass or composition. This theory has passed test after test here on Earth, but does it still hold true for some of the most massive and dense objects in the known universe, an aspect of nature known as the Strong Equivalence Principle? An international team of astronomers has given this lingering question its most stringent test ever. Their findings, published in the journal Nature, show that Einstein's insights into gravity still hold sway, even in one of the most extreme scenarios the Universe can offer.

Take away all air, and a hammer and a feather will fall at the same rate—a concept explored by Galileo in the late 1500s and famously illustrated on the Moon by Apollo 15 astronaut David Scott.

Though a bedrock of Newtonian physics, it took Einstein's theory of gravity to express how and why this is so. To date, Einstein's equations have passed all tests, from careful laboratory studies to observations of planets in our solar system. But alternatives to Einstein's general theory of relativity predict that compact objects with extremely strong gravity, like neutron stars, fall a little differently than objects of lesser mass. That difference, these alternate theories predict, would be due to a compact object's so-called gravitational binding energy—the gravitational energy that holds it together.

In 2011, the National Science Foundation's (NSF) Green Bank Telescope (GBT) discovered a natural laboratory to test this theory in extreme conditions: a triple star system called PSR J0337+1715, located about 4,200 light-years from Earth. This system contains a neutron star in a 1.6-day orbit with a white dwarf star, and the pair in a 327-day orbit with another white dwarf further away.

"This is a unique star system," said Ryan Lynch of the Green Bank Observatory in West Virginia, and coauthor on the paper. "We don't know of any others quite like it. That makes it a one-of-a-kind laboratory for putting Einstein's theories to the test."

Since its discovery, the triple system has been observed regularly by the GBT, the Westerbork Synthesis Radio Telescope in the Netherlands, and the NSF's Arecibo Observatory in Puerto Rico. The GBT has spent more than 400 hours observing this system, taking data and calculating how each object moves in relation to the other.

How were these telescopes able to study this system? This particular neutron star is actually a pulsar. Many pulsars rotate with a consistency that rivals some of the most precise atomic clocks on Earth. "As one of the most sensitive radio telescopes in the world, the GBT is primed to pick up these faint pulses of radio waves to study extreme physics," Lynch said. The neutron star in this system pulses (rotates) 366 times per second.

The Westerbork Synthesis Radio Telescope in the Netherlands was one of three telescopes used to study the three-star system, along with the Green Bank Telescope in West Virginia and the Arecibo Observatory in Puerto Rico. Credit: ASTRON

"We can account for every single pulse of the neutron star since we began our observations," said Anne Archibald of the University of Amsterdam and the Netherlands Institute for Radio Astronomy and principal author on the paper. "We can tell its location to within a few hundred meters. That is a really precise track of where the neutron star has been and where it is going."

If alternatives to Einstein's picture of gravity were correct, then the neutron star and the inner white dwarf would each fall differently toward the outer white dwarf. "The inner white dwarf is not as massive or compact as the neutron star, and thus has less gravitational binding energy," said Scott Ransom, an astronomer with the National Radio Astronomy Observatory in Charlottesville, Virginia, and co-author on the paper.

Through meticulous observations and careful calculations, the team was able to test the system's gravity using the pulses of the neutron star alone. They found that any acceleration difference between the neutron star and inner white dwarf is too small to detect.

"If there is a difference, it is no more than three parts in a million," said coauthor Nina Gusinskaia of the University of Amsterdam. This places severe constraints on any alternative theories to general relativity.

This result is ten times more precise that the previous best test of gravity, making the evidence for Einstein's Strong Equivalence Principle that much stronger. "We're always looking for better measurements in new places, so our quest to learn about new frontiers in our Universe is going to continue," concluded Ransom.

The pulsar and the inner white dwarf fall in the gravitational pull of the outer white dwarf (in red). In most theories of gravity, the very strong gravity of the pulsar means it will fall with a different acceleration than the inner white …more

Drop a marble and a cannon ball off the Leaning Tower of Pisa at the same time and they will hit the ground at the same time. That fact is explained by Albert Einstein's theory of gravity—general relativity—which predicts that all objects fall in the same way, regardless of their mass or composition.

Even the Earth and the moon "fall" in the same way toward the sun as they orbit each other.

Einstein's theory has passed all tests in laboratories and elsewhere in our solar system. But scientists know that quantum mechanics behaves differently, so Einstein's theory has to break somewhere. Does this principle also hold for objects with extreme gravity?

The answer is "yes," according to an international team of astronomers, including one from the University of Wisconsin-Milwaukee. They have tested the question with the help of three stars orbiting each other in a natural "laboratory" about 4,200 light years from Earth.

Findings from the team, led by researchers at the University of Amsterdam and the Netherlands Institute for Radio Astronomy (ASTRON), are published today in Nature.

Triple star system

Their test subject is a triple star system called PSR J0337+1715, consisting of a neutron star in a 1.6-day orbit with a white dwarf. This pair is in a 327-day orbit with another white dwarf farther away.

About the size of a planet, a white dwarf is a star that hasexhausted its nuclear fuel and only the hot core remains. While white dwarfs are small and dense, nothing beats the density of a neutron star, which is a cinder left over after a burned-out star has exploded. Its gravity has crushed the massive remains into a remnant the size of a city.

The neutron star becomes a pulsar when it spins rapidly and has a strong magnetic field. Pulsars emit radio waves, X-rays or even optical light with each rotation.

The researchers made the measurement just by tracking the neutron star, a pulsar.

"It rotates 366 times per second, and beams of radio waves rotate along," said Anne Archibald, the paper's first author at ASTRON and the University of Amsterdam. "They sweep over the Earth at regular intervals, like a cosmic lighthouse. We have used these radio pulses to track the position of the neutron star."

White dwarf gravity

When the pulsar moves, something is causing it, said David Kaplan, an associate professor of physics at the University of Wisconsin-Milwaukee and a co-author on the paper. "If Einstein is correct, it has to be the gravity of the white dwarf it's circling that's making the pulsar move."

The team of astronomers followed the neutron star for six years using the Westerbork Synthesis Radio Telescope in the Netherlands, the Green Bank Telescope in West Virginia and the Arecibo Observatory in Puerto Rico.

If the neutron star fell differently from the white dwarf, the pulses would arrive at a different time than expected. But as far as the researchers know, that didn't happen. Archibald and her colleagues found that any difference between the accelerations of the neutron star and white dwarf is too small to detect.

This system offers the researchers the opportunity to test the nature of gravity with much more sensitivity, said Kaplan, who was among the researchers who first published on the system that was discovered in 2012.

"We've done better with this system than previous tests by a factor of 10," said Kaplan. "But it's not an ironclad answer. Reconciling gravity with quantum mechanics is still unresolved."

Can't ignore relativity

A more precise description of gravity is important for other reasons too, said Kaplan.

"If you ignored general relativity but then tried to use the GPS on your phone, you'd end up far from your destination," he said. "But we're also trying to understand how the universe works here. We still don't understand how stars move."

Advancements in radio telescopes offer more chances at finding the perfect triple system to test, said Jason Hessels, associate professor at ASTRON and the University of Amsterdam.

If the Square Kilometre Array is built in Australia and South Africa as planned, it would be the largest radio telescope in the world, capable of finding many more millisecond pulsars as are now known in our galaxy.

"Among these yet undiscovered systems may lurk even more powerful tools for understanding the universe," Hessels said. "Perhaps one of these may provide our first peek at a theory beyond Einstein's."